Deep-ocean Bottom Pressure and Temperature Sensors Report: Methods and Data

This report documents ocean bottom pressure data collected from September 1983 to May 1985 in eleven deployments of pressure sensors under the Gulf Stream northeast of Cape Hatteras in depths of 3300 to 3900 m, as part of the Gulf Stream Dynamics Experiment. In past experiments, pressure sensors suitable for ocean depths have typically exhibited systematic drifts in calibration that seriously contaminate any observed periodicities longer than a few days. We used Digiquartz sensors (manufactured by Paroscientific, Inc.), because these sensors offered potentially much lower drift than other commercially available sensors. In these sensors, either a bellows or a Bourdon tube applies stress to an osciilating quartz-crystal beam, causing its oscillation frequency to vary. Several factors influence the amount of drift: bellows vs. Bourdon-tube construction, the applied pressure, the duration of deployment, and, for some sensors, high-pressure preconditioning in the lab. For the sensors deployed in the Gulf Stream, the total drift during deployments lasting from 3 to 12 months ranged from undetectable (≦0.01 dbar) to 0.20 to 0.50 dbar. About half of the total drift typically occurred within the first 6 days of deployment. We estimate the residual error in the final pressure records, after the dedrifting calculations, to be typically 0.02 dbar r.m.s. (or 0.06 dbar r.m.s.) if the first 6 days of the record are excluded (or included, respectively). This low drift-error opens many possibilities for studies that require knowledge of the low-frequency dynamic pressure signal in the deep ocean. Part I on Methods contains a short review of bottom pressure measurement in the deep ocean, a description of the sensors that we used, a discussion of their performance and drift relative to type of construction and prior pressurization history ( preconditioning ), and estimates of the accuracy of the dedrifted pressure records. In Part II of this report, the full data processing is described, including calibaration parameters, corrections for the influence of temperature variations on the pressure sensor, and parameterization to remove sendor drift errors by least-squares regression onto an exponentially decaying time-dependence. Time series are plotted which illustrate several steps in the processing: the edited half-hourly pressure records, the detided pressures with drift-model curves superimposed, and the low-pass filtered, dedrifted pressure records (i.e., after subtracting the estimated drift curve)

The SYNOP Experiment: Bottom Pressure Maps for the Central Array May 1988 to August 1990

This report documents our procedures and the accuracy of objective maps of bottom pressure in the Synop Central Array region between June 1988 and August 1990. The pressure maps are made by using multivariate objective analysis procedures to combine data from bottom pressure measurements by Inverted Echo Sounders (PIES) and bottom current meter measurements in the Central Array between June 1988 and August 1990. A key new feature is that all daily maps can be adjusted to a fixed reference level by combining measurements of bottom pressure and near-bottom currents, (Pb, u, v). While objective stream functions mapped from (u,v) alone are spatially consistent, the reference levels vary temporally from map to map. On the other hand, each bottom pressure record Pb has a temporally consistent reference but the reference can vary from site to site. The combination allows us to adjust all to a common, consistent reference level in multivariate objective maps. The pressure maps and error maps are displayed daily for 26 months with overlaid measured current vectors. Mapping procedures are documented along with error analyses and comparisins with measured fields. The results produce bottom pressure and current fields with typical error of only 2 mbar and 2 cm/sec, compared to typical signal standard deviation of 6 mbar and 9 cm/s. The report is one of a series of data reports that document aspects of our participation in the SYNoptic Ocean Predtiction (SYNOP) experiment

The SYNOP Experiment: Inverted Echo Sounder Data Report for Jun 1989 to Sep 1990

The SYNoptic Ocean Prediction experiment (SYNOP) was undertaken with the goal that increased understanding of the Gulf Stream obtained through coordinated observations could be integrated with numerical models, including predictive models of the Gulf Stream. Our moored experiment, which began in the fall of 1987, consists of two separate arrays in the Gulf Stream as part of the SYNOP program. The Inlet array of inverted echo sounders (IES) and deep current meters measures key parameters that that describe the variability of the Gulf Stream and the deep western boundary current (DWBC) near Cape Hatteras. In this region the Gulf Stream first flows into deeper water and crosses over the DWBC. The Central array of IESs, in a 350 km square centered on the Gulf Stream near 68 o W, monitors the thermocline structure of the Gulf Stream in the region of large meanders and frequent interactions with rings. The array also contained thirteen tall current meter moorings, that reached into the Gulf Stream core. Additionally most of the IESs in the interior of the array are outfitted with bottom pressure recorders (PIES). IES data recovered during the summer of 1990, from the Year 3 deployment period, are documented here by plots and tables of basic statistics and pertinent deployment information. Altogether 32 IES records are presented, plus pressure records at 12 sites. he echo sounders were recovered during a cruise aboard the R/V Endeavor, EN216 (4 August 1990 to 5 September 1990). The IESs had been deployed the previous summer during cruises on the R/V Oceanus, OC207 (26 May 1989 to 21 June 1989) and OC210 (8 August 1989 to 1 September 1989). One IES was exchanged in mid-October 1989 using the R/V Cape Henlopen. The plots are time series of measured travel time, pressure, temperature; the residual pressure; and low-pass filtered records of residual pressure, thermocline depth, and temperature. A brief description of the experiment is given; the standard steps of data processing are discussed along with special processing for several IES records that had different problems

Jet Streaks in the Gulf Stream

Mesoscale alongstream speed changes of the Gulf Stream are diagnosed from an array of current meters at depths 400, 700, and 1000 m, near 68°W, during the development of steep [ratio of “amplitude” to “wavelength” O(1)] meanders. Speed maxima (jet streaks) are generally found between trough and crest axes in steep meanders with local speed minima near the trough and crest axes. Speed changes along streamlines can be quite dramatic. Speed changes along the jet axis, between jet streaks and local minima in excess of 0.60, 0.40, and 0.35 m s−1, are observed at depth 400, 700, and 1000 m, respectively. This is in comparison with peak speeds in a frontal coordinates system mean of 1.22, 0.67, and 0.28 m s−1, at depth 400, 700, and 1000 m, respectively, from a previous study. The presence of the jet streaks can be explained kinematically as a superposition of the Gulf Stream and barotropic vortices. The development of these jet streaks in relation to the developing steep meanders differs from the canonical picture of jet streak/baroclinic wave development in the atmospheric jet stream in that the jet streaks in the Gulf Stream are predominantly fixed in place with respect to meanders as they steepen

Using the Inverted Echo Sounder to Measure Vertical Profiles of Gulf Stream Temperature and Geostrophic Velocity

The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A1, is presented. For a Gulf Stream IES record the so-generated A1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity

Inverted Echo Sounder (IES) Instrument Report

The Inverted Echo Sounder (IES) is an ocean-bottom moored instrument which accurately measures the time required for an acoustic pulse to travel from the sea floor to the ocean surface and back. The round-trip acoustic travel time varies in response to changes in the mean temperature profile of the water column above the instrument, as well as changes in water depth. The instrument is used as a sensitive indicator of changes in the main thermocline depth caused by synoptic-scale eddies or the shifting path of an ocean current. The IES is small (17 -diameter glass sphere) and self-contained, with its own acoustic release, relocation and recovery system. Its battery capacity and digital tape recorder (data capacity 107 bits) allow it to be deployed for up to 18 months. It can operate in water depths of up to 6700 m and requires only an anchor as mooring equipment. Microprocessor-based electronics allow programmable data formatting and sampling cycles. Additional data channels for pressure, temperature, and ambient noise are optional. The system has undergone extensive development and field testing, resulting in a reliable, cost-effective means to study temporal variability in large-scale features of the temperature field of the oceans

Gulf Stream Meanders: Observations on the Deep Currents

During 1979–1980, an array of inverted echo sounders (IES) and three deep current meter moorings were deployed on the continental slope 100–200 km northeast of Cape Hatteras, North Carolina. This array continuously monitored the path of the Gulf Stream and the deep currents under it. The mean currents at two sites 1000 m off the bottom near the northern edge of the stream were veered to the right of the mean surface path, indicating a deep inflow to the stream. Mean currents 500 and 1000 m off the bottom 50 km farther offshore were northeastward, nearly colinear with the surface Gulf Stream path. The deep velocity fluctuations are characterized by a transition from transverse flow aligned with the local bathymetry for periods longer than about 12 days to fluctuations with a cross-stream orientation for shorter periods. For periods between 4 days and 1 month, cross-stream movements of the Gulf Stream temperature front are vertically coherent and nearly barotropic, based on correlations between the IES-measured stream path and deep temperature fluctuations. Temperature fluctuations at the current meter sites lead cross-stream (positive onshore) velocity fluctuations by approximately 90°. Consideration of the nondiffusive fluctuating heat equation for deep layers suggests a three-term balance between local rate of change, cross-stream horizontal advection, and vertical advection of temperature, with the first two being of like sign. Kinematically, this requires |wTz| \u3e |υTy|, so that parcel trajectories in the cross-stream plane are inclined at angles steeper than the mean cross-stream slope of the isotherms. Eddy energy conversion between the fluctuations and the mean field in deep layers is predominantly baroclinic, with e-folding growth time scales of approximately 10 days

The Gulf Stream Dynamics Experiment: Inverted Echo Sounder Data Report for the April 1983 to June 1984 Deployment Period

The Gulf Stream Dynamics Experiment was conducted in the region just northeast of Cape Hatteras from September 1983 to May 1985 to study the propagation and growth characteristics of Gulf Stream meanders. Data collected as part of the field experiment included inverted echo sounders, current meter moorings, and AXBT survey flights. This report documents the inverted echo sounder data collected from September 1983 to June 1984, as well as additional measurements made from April to September 1983. Time series plots of the half-hourly travel time and low-pass filtered thermocline depth measurements are presented for twenty-two instruments. Bottom pressure and temperature, measured at seven of the sites, are also plotted. Basic statistics are given for all the data records shown. maps of the thermocline depth field in a 240 km by 460 km region are presented at daily intervals

Internal Tides in the Southwestern Japan/East Sea

This paper investigates the internal tidal energy distribution in the southwestern Japan/East Sea using vertical round-trip travel time (τ) data from 23 pressure-sensor-equipped inverted echo sounders (PIES). The τ records are analyzed by bandpass filtering to separate time-dependent variability of the semidiurnal and diurnal bands. The semidiurnal internal tides exhibit a horizontal beam pattern of high energy, propagating into the open basin. They originate from a restricted portion of the shelf break where the Korea Strait enters the Ulleung Basin. The generation appears to occur at ∼200-m water depth near 35.5°–35.7°N and 130°–131°E, where the slope of bottom topography matches that of the wave characteristics, coinciding with the location where the semidiurnal barotropic cross-slope tidal currents are strongest. Maximum vertical displacement of the thermocline interpreted as a long-wave first baroclinic mode from the measured τ is about 25 m near the generation region. Annual and monthly variations of the propagation patterns and generation energy levels are observed, and these are closely associated with changes in the mesoscale circulation and stratification. Eastward (westward) refraction is observed when a warm (cold) eddy crosses the path of internal tide propagation. Moreover, when the generation region is invaded by cold eddies that spoil the match between shelf break and thermocline depth, the internal tidal energy level decreases by a factor of about 2. A simple geometric optics model is proposed to explain the observed horizontal refraction of the beam of semidiurnal internal tides in which stratification and current shear play essential roles. In contrast, diurnal internal tides are observed to be trapped along the continental slope region around 36°N

On Gulf Stream meander characteristics near Cape Hatteras

From 1979 to 1982, Gulf Stream path fluctuations within 375 km downstream of Cape Hatteras, North Carolina, have been monitored by inverted echo sounders. Histograms of the north wall locations along four cross‐stream sections change shape and increase in range dramatically downstream through the study area: Near Cape Hatteras, the histogram is peaked with the Gulf Stream found over half of the time within a narrow 10‐km range. The distributions become progressively more symmetric and the ranges widen downstream such that in the eastern portion of the study area the Gulf Stream can be found with equal probability throughout its 145‐km excursion range. From the 36‐month‐long time series near 73°W there is evidence of a seasonal cycle of Gulf Stream positions; northerly locations occur in the summer/fall, when transports are lower, and southerly locations occur in the winter/spring, when transports are higher. An observational dispersion relationship is presented for meander propagation and growth: Downstream propagation rates increase smoothly from about 14 km d−1 for meanders with periods and wavelengths (33 days, 460 km) to over 45 km d−1 for the (4 days, 180 km) meanders. Meander amplitudes show rapid growth rates in two separate bands, near (4–5 days, 180–230 km) and (10–33 days, 300–500 km)